1. Field of the Invention
The present invention relates to a dual antenna unitized by installing in parallel a patch antenna capable of transmitting and receiving a circularly polarized electromagnetic wave to and from a Global Positioning System (GPS) satellite or the like and a rod-shaped antenna capable of transmitting and receiving a linearly polarized electromagnetic wave for use in mobile telephony or the like.
2. Description of the Related Art
Vehicle-mountable patch antennas have been known since years ago as means for finding one's current geographical position by receiving an electromagnetic wave from a GPS satellite, and more recently vehicle-mountable dual antennas, each combining a rod-shaped antenna for mobile telephone use with a patch antenna for GPS use, have become available for practical use. Such a dual antenna has a common radome compactly housing a patch antenna and a rod-shaped antenna. As it can be handled as a relatively small antenna unit combining the function to receive a circularly polarized electromagnetic wave from a GPS satellite and a function to receive a linearly polarized electromagnetic wave for use in mobile telephony, the dual antenna is expected to find use in many different areas along with the development and expansion of information and communication services.
FIG. 4 shows a plan of such a dual antenna according to the prior art, in which the illustration of its radome is dispensed with. As illustrated, in a known dual antenna 1, a patch antenna 3 and a rod-shaped antenna 4 are provided in parallel on a base plate 2, and these two antennas 3 and 4 receive power supply via cables 5 and 6 and connectors (not shown).
To schematically describe its configuration, the patch antenna 3 is fabricated by providing a power feed patch 7, which is a radiation element of a microstrip structure, over a dielectric substrate 8, mounting this dielectric substrate 8 on a circuit board 9, and packaging components of a low-noise amplifier circuit (not shown; including an amplifier and a band pass filter), covered by a shield case, on the bottom side of the circuit board 9. For the power feed patch 7, a short axis 7a having a smaller resonance length matching a higher resonance frequency (f1) mode and a long axis 7b having a greater resonance length matching a lower resonance frequency (f2) mode are defined by providing degenerate separating elements such as notches or projections, and the short axis 7a and the long axis 7b orthogonally cross each other. Excitation at a frequency between these high and low resonance frequencies f1 and f2 to generate a 90-degree phase difference between the radiation fields in the two modes can make the composite radiation field a circular polarization. This enables a circularly polarized electromagnetic wave of 1.575 GHz transmitted from a GPS satellite to be received by the patch antenna 3. When it is received, of the electromagnetic wave higher in frequency than the resonance frequency f1, field components mainly in the direction of the short axis 7a are received and, of the electromagnetic wave lower in frequency than the resonance frequency f2, field components mainly in the direction of the long axis 7b are received. To add, as the power feed patch 7 is connected to a low-noise amplifier circuit via a power feed pin 10, signals received by the power feed patch 7 are delivered to a GPS receiver circuit (not shown) via the cable 5 and the like in a state in which they have been amplified by the amplifier and cleared of signals in the undesired frequency band by a band pass filter.
On the other hand, the rod-shaped antenna 4 is fixed to a circuit board 11 adjoining the patch antenna 3, and erected in a slightly inclined state. This rod-shaped antenna 4 can transmit and receive electromagnetic waves of the 850 MHz band and the 1.85 GHz band used for mobile telephony, and is connected to a transmitter/receiver (not shown) via the cable 6 and the like. The radome (not shown) is formed in such a shape as covers the patch antenna 3 and the rod-shaped antenna 4 and fixed to the base plate 2.
Incidentally, in the GPS reception unit of the dual antenna of the example of the prior art described above, if any undesired signal whose frequency is other than that of the desired receive signal is received by the patch antenna 3, this undesired signal is usually removed by the band pass filter, and therefore it rarely interferes with the desired signal. However, if this undesired signal is a powerful electromagnetic wave for mobile telephony radiating from the adjoining rod-shaped antenna 4, there may arise a problem that the low-noise amplifier circuit is saturated with signals and invite interference.